1. Field of the Invention
This invention relates to electrical insulation, and more particularly to an overcoated bulky sleeve and a method for electrical insulation of electrical conductors using said sleeve.
2. Discussion of the Art
The various functional devices and/or circuits within an electrical apparatus are generally electrically interconnected by elongated electrical conductors (e.g. wires, bus bars, cables, etc.) which are tupically electrically insulated. Electrical insulation serves to isolate the conductor from potentially disruptive environmental factors of a mechanical, electrical or chemical nature thereby insuring that current flows when and where desired without interruption.
Electrical insulation is generally provided either before or after interconnection of the conductor within the electrical apparatus. Prior to interconnection, the insulation material, generally polymeric in nature, may be coated or extruded directly onto and around the elongated conductor. Such insulation is generally partially removed, usually from the extrmities, as an interconnection is made to or within the electrical apparatus. Alternately, bare conductors or exposed interconnection points of otherwise insulated conductors may be insulated after interconnection by coating (for example, with a polymeric material), wrapping with an insulating tape, fitting with an insulating sleeve or cap, or impregnating a porous tape or sleeve with an insulating material such as a polymeric material either before or after positioning it on the conductor.
An electrical motor is one example of an electrical apparatus having electrical conductors which interconnect the various functional devices and/or circuits within the apparatus. The coil leads and the phase connectors which connect the quadrants of the coils of the motor are examples of electrical conductors within the motor which require insulation.
The stator of the motor is generally insulated by resin impregnation applied by a vacuum/pressure impregnation process, and is preferably void-free. Resin is forced into the interstitial areas by alternately pulling a vacuum, then applying pressure. Before the stator is impregnated, the motor is generally assembled and pretested. Conductor insulation must be in place during pretest where short term voltages on the order of 6,000 to 8,000 volts may be applied for on the order of a microsecond. Insulating sleeves which are deformably expandable and can be pulled-back along the length of the conductor so that permanent interconnections can be made subsequent to the temporary connections of pretest, are (for this application and many other applications) generally preferred to tape or other types of retrofitted insulation. The interiors of insulation sleeves associated with the conductors of the stator are generally resin impregnated at the same time that the stator is impregnated. Sleeve impregnation is also preferably void-free, but resin outflow due to gravity before the resin cures is known to occur and to result in an imperfect impregnation, especially where low viscosity resins are used for penetration of complex structures.
An electrical motor also provides a good example of the possible mechanical, electrical or chemical environmental forces which can cause insulation failure, even failure of insulation sleeves which offer many obvious design advantages over other types of post-interconnection conductor insulation. Insulation failure may be due to mechanical forces induced by electromagnetic torque or to vibration-induced wearing-away of the insulation against other components of the electrical apparatus. Thermal expansion and contraction may cause wear. Localized overheating due to the presence of air or vapor pockets which reduce heat transfer may accelerate thermal degradation of the insulation materials and induce stress cracking. Oxygen or ozone from electric discharges may enter and cause oxidative degradation of polymeric materials. Water may enter and corrode metal parts or short circuit conductors.
Electrically insulating sleeves are tubular structures fabricated from electrically insulating materials. The simplest sleeve is a one-component tube composed of, for example, a polymeric material, a ceramic material, or an interthreaded yarn (i.e. a braid, a knit, or a woven and sewn fabric). The insulation value and wear-resistance of insulating sleeves may be improved by providing a dense, thick-walled sleeve or a plurality of nested sleeves of varying diameters. Alternately, composite sleeves may be employed such as a braided fiberglass sleeve overcoated with a polymeric material or a plurality of overcoatings of polymeric materials.
The insulation value and wear-resistance of a sleeve may be further improved by impregnation of a porous sleeve with an impregnation composition, for example, a resin, prior to installation of the sleeve around a conductor. After installation the resin-rich pre-impregnated sleeve is compressed against the conductor thereby forcing resin out of the sleeve to improve contact with the conductor. Alternately the interstices of the sleeve and/or between the sleeve and the conductor may be impregnated with an impregnation composition, such as by means of the previously discussed, well-known vacuum/pressure impregnation process wherein the impregnation composition is forced into the interstitial areas by alternately pulling a vacuum, then applying pressure.
Sleeves are generally selected to have the smallest possible diameter that will fit over the conductor to be placed therewithin, thereby improving the electrical insulation value and wear-resistance by improving the contact of the sleeve with the conductor. Bulky sleeves having a bulky interior, such as a braided asbestos sleeve, are known and have been used for electrical as well as thermal insulation. These provide a more intimate fit with the conductor therewithin.
Staple fiber yarn braids are more difficult to manufacture than continuous filament yarn braids because of their tendency to fibrillate even through spun into a filament. Staple fibers are fibers of shorter length, whether natural or man-made. For example, staple fiberglass is 6 to 15 inches long and staple asbestos is up to 12 inches long. All natural fibers except for silk are staple fibers.
The spun filaments of natural fiber staple yarns have a high intrinsic surface area compared to man-made continuous filament yarns. Natural fiber yarns therefore tend to absorb and/or to surface absorb more water than man-made continuous filament yarns. This characteristic detrimentally influences their dielectric properties and renders them less desirable for electrical insulation sleeving, even when such sleeving is overcoated. Moreover, surface water must be removed before a natural fiber yarn sleeve is impregnated with most impregnation compositions.